I had an acute a-Fib episode at 3 weeks post-op, including slowly and gently collapsing onto the bathroom floor, after feeling weak and miserable for a few hours. I'd had a flu cough and fever for several days, and it seemed to have cleared up, then I started feeling like death warmed-over, and eventually collapsed. My HR was 150. After a few hours prep, I rode an ambulance to the ER of the hospital where I had my AVR. For me, a shot of a calcium beta-blocker dropped my HR to normal immediately, but I was still in A-flutter. They put me on Metoprolol 25mg x 2/day (which I think I should have been on continually post-op, long story), which seemed to solve the problem. At 3 months post-op, after a 48-hour Holter session, I weaned off the Meto (and the Warfarin). So far, so good, starting to feel more like a "normal person" than a "heart patient", YAY!
I wish you a similar resolution.
Unfortunately, the cardiologist in charge of my Cardiac Rehab program told us that cardioversion almost never solves an A-fib problem permanently, though somebody posting above has had multi-year success. A show of hands in our Rehab group showed 4 people who'd had cardioversion for A-fib, and 3 of them had the A-fib return after a few months. The exception was only a few months post-cardioversion, and the snarky Cardio said "You're still young!" (Nice!)
BTW, I was warned DECADES ago that holding my breath and pushing hard when I'm really constipated (almost never, he says thankful for small favors!) is a very bad idea, because of its effect on blood pressure and heart-attack risk. (Apparently many MIs happen on the can.) So I wouldn't rush to follow Rob's "4. You can also strain real hard as though you were constipated." At least not until getting a second opinion.